In 35:5-6 the representatives of the tribe of Menashe come complaining that by giving Tzlafchad's portion in Israel to his daughters, they are hurting the tribe of Menashe - the girls will marry out, and the land will follow them to the husbands tribe.
Moshe agrees and makes a stipulation that the girls should only marry within the tribe of Menashe.
Why did Moshe not respond to them that it is too late - they should have argued this when he first brought the issue up before God? Nobody mentioned such an issue then, so what right do they have to limit the girls now after the case has already been decided and closed?
A person has to not be afraid to do the right thing. Even if that means re-visiting something previously decided. Even if it means looking at something previously discussed in a new light. If something was done unjustly, it should be fixed. The wrong must be righted. If there is a way to do so, if the wrong can possibly be righted, even if it seems unfair, like to impose conditions on Tzlafchad's daughters well after they were given their freedom, a person must have the courage to step in and right that wrong.
That is what Moshe did. Yes, he had already told the girls the inheritance is theirs. Now he heard of a new issue and had to find a way to make sure his previous decision did not hurt the greater community, and that required imposing a new condition. It is never too late to right a wrong.